1. Field of the Invention
The present invention relates to an accumulator which is used as a pressure accumulating apparatus, a pulse pressure damping apparatus or the like. The accumulator in accordance with the present invention is used, for example, in a hydraulic piping or the like in a vehicle such as a motor vehicle or the like.
2. Description of the Conventional Art
Conventionally, there has been known an accumulator structured such that a bellows is arranged in an inner portion of an accumulator housing provided with an oil port connected to a pressure piping and an internal space of the housing is comparted into a gas chamber in which a high pressure gas is charged and a liquid chamber communicating with a port hole, and the accumulator includes a type that an inner peripheral side of a bellows 51 is set to a gas chamber 55 and an outer peripheral side is set to a liquid chamber 56 by fixing the other end (a fixed end) 51b of the bellows 51 in which a bellows cap 52 is attached to one end (a floating end) 51a to an end cover 54 on an upper portion of a housing 53 as shown in FIG. 8 (which is called as “inside gas type” since the gas chamber 55 is set to the inner peripheral side of the bellows 51, refer to Japanese Unexamined Patent Publication No. 2005-315429.), and a type that the outer peripheral side of the bellows 51 is set to the gas chamber 55 and the inner peripheral side is set to the liquid chamber 56 by fixing the other end (the fixed end) 51b in which the bellows cap 52 is attached to one end (the floating end) 51a to an oil port 57 in a lower portion of the housing 53 as shown in FIG. 9 (which is called as “outside gas type” since the gas chamber 55 is set to the outer peripheral side of the bellows 51, refer to Japanese Unexamined Patent Publication No. 2001-336502 or Japanese Unexamined Patent Publication No. 2007-187229).
In this case, in the accumulator connected to a pressure piping of a device, a liquid (an oil) is discharged little by little from a port hole 58 if an operation of the device is stopped, and in the outside gas type accumulator in FIG. 9 mentioned above, the bellows 51 is contracted little by little accordingly by the charged gas pressure, a seal 59 provided on a lower surface of the bellows cap 52 comes into contact with the other member 60 so as to come to a so-called zero-down state. Further, in this zero-down state, since a part of the liquid is sealed within the liquid chamber 56 (a space between the bellows 51 and the seal 59) by the seal 59, and pressure of the sealed liquid balances with the gas pressure of the gas chamber 55, it is possible to inhibit excessive force from being applied to the bellows 51 so as to generate an abnormal deformation.
However, in the case that the zero-down due to the operation stop is carried out at a low temperature, and the temperature rises in this state, the liquid sealed in the liquid chamber 56 and the charged gas are thermally expanded respectively, and the respective pressures rise. In this case, a rising rate of the pressure is higher in the liquid than in the charged gas, however, since a pressure receiving area in the bellows cap 52 is set smaller than the charged gas, the bellows cap 52 does not move until the liquid pressure becomes considerably higher than the gas pressure. Accordingly, there is a case that a great pressure difference coming to about some MPa is generated between the liquid pressure and the gas pressure in the inner and outer sides of the bellows 51, and if the great pressure difference is generated, there is a risk that the bellows 51 is abnormally deformed or the seal 59 is damaged.
Further, since an accumulator shown in FIG. 10 is an outside gas type accumulator similarly to the accumulator in FIG. 9, and has a peculiar structure that an auxiliary liquid chamber 71 is provided in an inner peripheral side of the bellows 51, and a piston 72 with a piston seal 73 is inserted into the auxiliary liquid chamber 71 so as to make a stroke motion, the following disadvantages are pointed out (refer to Japanese Unexamined Patent Publication No. 2003-278702).
(i) An expansion of the bellows 51 can be carried out only within a range of a volumetric capacity of the auxiliary liquid chamber 71 (a contraction of the bellows 51 is limited if the volumetric capacity of the auxiliary liquid chamber 71 is increased, and a liquid amount for expanding the bellows 51 becomes small if the chamber 71 is made small, so that it is impossible to increase an amount of expansion).
(ii) Since the piston makes a stroke motion in a state in which the piston 72 is sealed with the piston seal 73, slip resistance due to seal surface pressure is great, and a motion of the bellows 51 slows down correspondingly to a loss thereof (a function as the accumulator is lowered).
Further, in Japanese National Publication of Translated Version No. 2005-500487, there is disclosed an accumulator structured such that a secondary piston is coupled to a bellows cap via a secondary bellows, however, the following disadvantage is pointed out in this conventional art.
(iii) Since a constraction of the bellows is generated in a state in which the secondary bellows is expanded at a time of zero-down, and the constraction of the bellows stops at the stage that the secondary piston reaches the lowest surface, it is impossible to secure a sufficient expansion and contraction stroke of the bellows.